Determining Loads on a Water Foil?

[CPosner]

I've got a rudder/fin that I am trying to determine the loads of to do an accurate FEA on.

I'm going to try to describe the situation, the craft is moving along the x-direction in water and the foil serves to keep the craft from "slipping" in the y-direction(normal to x) Therefore, foil aligned with x-axis

Simply speaking, there is an inherent load in the y-direction that needs to be in equilibrium. The load is transferred to the fin to keep the craft moving along x. How can I determine the reaction in the Y direction?

They way I am thinking is that there is the payload(m*g) in the Z-axis in the craft which can be broken down into local x and y components(relative to the Z) if the craft tilted at an angle relative to the y-axis(i.e. rotated about x-axis). Then I use the derived component as my load on the foil.

To me it seems "too" simple.

Are these assumptions correct or close? Any input is appreciated.

Thanks!

 

[rb1957]

as i understand it, a rudder develops a usefull side-force (usefull 'cause it trims out the boat) and this in turn causes a not-very-usefull couple (not-very-usefull 'cause it doesn't do much for us !) which is reacted by the tiller input (a much smaller force on a much longer arm).

"How can I determine the reaction in the Y direction?" ... it's equal to the applied load (yeah, i know not-very-usefull !) ... the side-force on the rudder is caused by the moment between the side-force on the boat (from the water, mostly on the keel) and the side-force on the sails ... the rudder works like the tailplane on a plane, creating a small balancing force.

In short, i think your suggestion is very simplified.

 

[CPosner]

Ahh, This craft is for surf/wave riding. So sails, no tiller, no keel.

I assume the load from the wave pushing toward shore is negligible because the board is moving with the wave, therefore no direct resistance with the wave itself. But, the foil must support the payload/rider...Which is where I get my assumptions from. Maybe that's more clear?

Thanks

 

[zerosum]

It seams to me that if what you are trying to analyze is something like a skeg on a surfboard, dynamic forces and moments will predominant over the static ones due to manuvering, whether the foil is steerable or not.

 

[CPosner]

Zerosum,

Determining those forces is my challenge...There are other designs out there so I can base sizes from that, but I would have piece of mind knowing the loading so I can do my own analysis.

 

[gwolf2]

Tricky. Perhaps the best way to tackle this is to ignore what loads are up top and take an extreme case.

The loads on the fin are always limited by the craft speed, water density, and fin angle relative to the water flow. This is why yachts flip over if you try to push it too hard. You could make a worst case scenario by assuming that you are at maximum speed and then suddenly turn 90 degrees.

Then the load will be 1/2 x rho x v**2 * A etc at the center of area of the fin.

This doesn't solve the interesting question of loads under normal conditions but it does give you something to work with. In reality the fin will be riding at some angle much lower than 90 degrees and acting like a wing. If you had some feel for that angle by watching other boats you could do a better analysis based on simplified wing theory. Even then I guess that the worst case is still a sudden change of course at max speed.

Hope this helps.

gwolf2